DIVISION OF STEMS. 



67 



ascends through it. The perfect, or heart-wood, is darker than the 

 sap-wood and is the principal part of the tree, as used for timber, being 

 the annual hard concentric layers and longitudinal woody fibres. The 

 age of a branch is determined by the layers at the base of each. In 

 the increase upwards of a tree, the sap having extended as far as pos- 

 sible, forms there a cone, at the summit of which a bud is formed. 

 From this springs a new shoot ; and a new layer of alburnum is, of 

 course, organized, which becoming perfect wood the following year ; 

 another new bud is again formed on the cone of the last ; and so on, 

 one annually encompassing another. 



Plants are distinguished on account of different modes of growth, 

 as mono-cotyledonous, arising from seeds with one cotyledon or lobe, 

 and di-cotyledonous, those from seeds with two cotyledons or lobes. 

 The first are also called endogenous (inside growers) the wood augment- 

 ing annually by internal additions to their centre, thereby pushing to the 

 surface previous annual formations. The second are also called exo- 

 genous (outside growers) the woody matter being increased annually 

 by external additions. All trees and shrubs of the U. S. except the 

 few palms of the south and some grasses, as the sugar cane, Indian 

 corn, etc. are of this latter kind di-cotyledons or exogenous. 



This cut represents the formation and structure of 

 the outside growers, which constitute most northern 

 plants, and shows the concentric circles and medullary 

 rays radiating from the centre. 



These are important distinctions in vegetable phy- 

 siology, and are of recent origin. 



The mono-cotyledons, or endogenous plants,- seldom have any bark 

 distinct from the other texture ; no liber or alburnum 

 in concentric layers, nor medullary rays, as the pith 

 extends almost to the circumference', instead of being 

 confined to the centre, as with di-cotyledons. The 

 number is however small here, as we have stated. 

 The cut is a representation of the internal structure 

 of this species of plants. 



The appearance of the wood shows it to be composed of longitudinal 

 fibres. Each appears to vegetate separately, and the principal part 

 of them seem to be pressed outward by new ones in the centre. The 

 old fibres thus pressed, are thick and compact near the exterior. 



It is apparent that this mode of growth is favorable to the extension 

 of plants logitudinally, i. e., for a straight, slim and uniform size, as 

 we find the palms, sugar cane, etc., of the tropical climates to be ; but 

 it is unfavorable to their growth in diameter. The roots of this kind 

 of plant are mostly fibrous. r ._._. 



Jl sectional view of the stem shows the cellular fMl 

 tissue to occupy three-fourths of it, and to be en- jiffi?T: 

 croaching upon the vascular system near the exteri- pyb ''!''k l! ;!"'' 



